Na⁺_i dependence of wild-type and G99R pumps. (A and B) Representative current recordings at 0 mV from giant inside-out patches excised from an oocyte expressing wild type (A) or one expressing G99R (B). The patches were perfused with intracellular solutions of varying [Na⁺_i] (a mix of Na⁺_i and K⁺_i solutions; Materials and methods). Application of 4 mM ATP_i induced outward pump currents in a [Na⁺_i]-dependent manner. Vertical deflections represent 25-ms voltage steps. (C) Mean [Na⁺_i]-dependence of ATP-induced currents normalized to the I_max from Hill fits to the mean data for wild type (squares, n = 5) and G99R (circles, n = 4) with parameters K_0.5 = 13.1 mM, n_H = 2.9 for wild type and K_0.5 = 33.1 mM, n_H = 1.2 for G99R (mean from fits in individual experiments are shown in the text). Mean ATP-induced current was 12.4 ± 2.6 pA in 50 mM Na⁺_i 90 mM K⁺_i for wild type and 12.4 ± 1.3 pA in 125 mM Na⁺_I for G99R.